Scroll type positive fluid displacement apparatus typically include parallel plates having involute wrap elements attached in intermeshed, angular relationship. The axes of the wrap elements are normally parallel and offset such that their relative orbital motion causes pockets of fluid defined by flank surfaces of the wrap elements and the end plates, to move between an inlet port and an outlet port.
Depending upon the configuration of the involute wrap elements and the relative direction of their orbital motion, a scroll machine may function as an expander (vacuum pump), a compressor, or a liquid pump. When used as an expander, the pockets of fluid moving through the machine originate near the center of the involutes and expand in volume as they move outward around the wraps. Conversely, in a scroll compressor, pockets of fluid move inward around the scroll wraps to a center discharge port, experiencing a substantial reduction of their volume in the process. In a liquid pump, each of the involute wraps makes only a single loop about the central axis such that the pockets of liquid are not subjected to a significant change in volume, but are simply forced to move around the scroll between an inlet port and a discharge port, exiting at a higher pressure.
The operating efficiency of a scroll machine is particularly dependent upon the effectiveness of the seal between the flank surfaces of the involute wraps in the radial direction, and between the tip of the wraps and the facing end plate in the axial direction. For applications where effective radial sealing is less important, a small clearance may be maintained between the flank surfaces of the intermeshed scrolls such that they do not contact each other. This design has been referred to as a "fixed-crank" type scroll machine, an example of which is disclosed in U.S. Pat. No. 4,082,484. In a more common approach generally providing higher operating efficiency, the flank surfaces of the intermeshed wrap elements are caused to contact each other with sufficient force to achieve radial sealing. A scroll machine so configured is thus conveniently referred to as a "radially compliant" type.
U.S. Pat. No. 3,924,977 discloses a radially compliant linking means for linking a driving mechanism to an orbiting scroll member. In the '977 patent, two embodiments are shown for providing radial compliance--a swing link, and a sliding-block linkage. In the first, the axis of the swing link is perpendicular to the eccentricity radius of the orbiting scroll and comprises a connecting rod mounted on a drive shaft crank pin, a bearing assembly, and a spring loaded assembly arranged to partially offset the centrifugal force on the orbiting scroll member when the drive mechanism is turning at normal operating speed. At start-up, the centrifugal force on the scroll member is nil and the centripetal force applied by the spring causes the scroll member to orbit with an eccentricity radius less than normal. The drive mechanism, e.g., electric motor, does not have to develop significant torque upon start-up, since the flanks of the involute wrap elements are initially not in sealing contact.
In another U.S. patent application, Ser. No. 195,289, assigned to the same assignee as the present application, a scroll type machine is disclosed having a swing link to effect radial compliance, with counterweight means to provide an offsetting centripetal force to the centrifugal force on the orbiting scroll member. Unlike the spring loaded assembly of the '977 patent, the counterweighted swing link disclosed in this application insures adequate radial sealing between the flanks of the scroll members, independent of the operating rotational speed of the machine. The application discloses a coiled spring captive within the swing link which acts against a post on the end of the drive shaft to bias the flank surfaces of the involute wraps slightly apart when the apparatus is at rest, thereby reducing the starting torque required of the driving mechanism.
A potential problem exists with regard to the prior art swing link designs discussed above. The pivot pin bearing in the swing link may be subject to fretting due to the relatively high loads and small relative motion to which it is exposed. The limited motion of the swing link about the pivot pin is conducive to fretting because wear particles tend to become trapped in the load bearing area rather than being carried out, as would be the case if there was substantial motion between the swing link and pin, e.g., if the swing link rotated about the pin.
In view of these potential problems with the swing link design disclosed in the prior art, it is an object of this invention to provide an alternative swing link with radial compliance, having the means to couple the drive shaft to the swing link and to bias the wrap elements apart in one element.
It is a further object of this invention to eliminate the pivot pin and bearing to avoid potential fretting problems.
A still further object is to simplify the radial compliant swing link.
These and other objects of the invention will be apparent from the description of the preferred embodiment which follows, and from the attached drawings.